Low power battery supervisory circuit with adaptive battery health monitor

We propose a battery supervisory circuit (BSC) for wireless sensor nodes that automatically adapts to varying battery health, as reflected by its internal resistance (R_BAT), and establishes a constant effective threshold voltage. Compared to a conventional fixed-threshold BSC, the new design avoids oscillation and widens the usable range of battery voltages, independent of R_BAT. R_BAT is measured by inducing a test current using decaps and measuring the resulting battery RC response time. When tested with a 2μAh battery and 11μA sensor system, the BSC reduces the required hysteresis from 656mV to 77mV, increasing the usable battery voltage range by 2.7×.